1. Field of the Invention
This invention relates to an improved apparatus and process for fabricating a coated substrate with a cured, polymerized coating. More particularly, it relates to an apparatus and process in which a coating of polymerizable or polymeric material is applied to the substrate in a vacuum liquifiable solvent, and the solvent is evaporated and recovered in an improved manner. Most especially, the invention relates to an apparatus and process for applying lacquer or enamel coatings to a tinplate substrate.
2. Description of the Prior Art
It has long been conventional practice to coat the inside of cans formed from tinplated metal used for food and other packaging with a lacquer or enamel in order to prevent corrosion of the cans from the food product and contamination of the food product. As typically practiced, such lacquer or enamel coatings are roll applied in a solvent to flat stock used to make the can bodies, the solvent is evaporated in air in a heated oven, and the flat stock is further heated to cure the lacquer or enamel. As concerns over environmental pollution from such solvents have increased, the practice of incinerating the evaporated solvent has begun. Solvent recovery has not proved to be practical in such processes, because the concentration of solvent in the air from such ovens is kept below about 0.5 percent by weight, in order to avoid creating explosive
After the coating of the flat tinplate stock has been carried out in this manner, the can bodies are formed, sealed along the seam, flanged for attachment of a top and bottom, and beaded to increase their strength. These operations produce scratches and other visible damage to the lacquer or enamel coatings. Additionally, such mechanical stresses applied to the coatings weaken them in ways that are not readily detectable by visual observation, such as by stretching the polymers which form the coatings. The resulting loss of integrity of the coatings results in accelerated deterioration of the cans after they have been filled and causes undesirable contamination, particularly in the case of beverages, the flavor of which is very sensitive to minute amounts of impurities.
It has generally not proved to be economical to coat the interior of the can bodies after the mechanical forming and seam soldering or welding processes have been completed in such air oven heating processes because of the much higher oven volume occupied by formed can bodies as compared with flat metal stock. However, a second layer of lacquer or enamel is often applied at the can seam, but only partially cured, as a way of increasing the integrity of the coating inside the finished can. Some seamed beer cans have a second vinyl coating applied after formation of the can body because beer is especially sensitive to flavor alteration by contaminants, but such a second coating adds materially to the cost of the cans.
The use of air ovens for solvent evaporation and the curing of the coatings further introduces problems in coating integrity from cratering and trapped air or other gas around can seams. This is especially true if the thickness of the cured coating is not carefully controlled and kept thin.
In recent years, as energy costs have escalated rapidly, there has been a demand for a more energy-efficient, non-polluting process for providing such coatings on tinplate cans and similar substrates. There is further a desire to increase coating integrity, especially at different thicknesses.
My issued Israeli Patent based on Application No. 50398, filed Sept. 1, 1976, discloses vacuum treatment apparatus in which a vacuum condensable gas is provided to exclude air at the entrance and exit of a vacuum chamber in which materials are subjected to various treatment processes. The vacuum condensable gas is provided separately from the materials being treated in the vacuum chamber.